Yes, get the good test kit.
If the "sand" poofs up into clouds when disturbed - it is dead algae. Currently you likely are growing more, so an endless cycle of filtering it out, and then more replacing it.
Do as Texas Splash advised right now. When your kit arrives, the critical numbers will be the CYA (stabilizer), Free Chlorine, and Combined Chlorine. For the Cl, use the test involving powders and drops.
Visit your local WalMart and get several gallons of Pool Essentials liquid chlorine. Once we know the numbers, we can guide you on how much to add, and how often to test, to permanently knock it all down. It will be a sustained effort, not a one time event. Read up on the SLAM method in the Pool School here.